1. Field of the Invention
The present invention relates generally to the fields of molecular biology of parasitic bacteria, particularly agents of rickettsia type diseases and ehrlichial bacteria. More specifically, the present invention relates to molecular cloning and characterization of Ehrlichia canis 120-kDa immunoreactive protein gene.
2. Description of the Related Art
Ehrlichia spp. are obligately intracellular gram negative bacteria which reside in the endosome of hematopoietic cells and infect various animal hosts including humans, domestic and wild canidae, deer, horses, sheep, cattle, and wild rodents. Each member of tribe Ehrlichieae has its own particular target cell tropism. Most species of Ehrlichia are either monocytotropic (E. canis, E. chaffeensis, E. sennetsu, E. risticii, and E. muris) or granulocytotropic (human granulocytic ehrlichia, E. equi, E. phagocytophila, and E. ewingii) with the exceptions of Cowdria ruminantium which grows in the endothelial cells of the host and Anaplasma marginale, a red blood cell parasite.
Although ehrlichiae were described in the early part of this century, they received very little attention because they were considered pathogens of only veterinary importance in the United States until this decade. The renewed interest in ehrlichiae is due to the emergence of ehrlichioses affecting humans. In the last decade two new human Ehrlichia pathogens (E. chaffeensis and a human E. phagocytophilia-like organism) were discovered in the United States (Bakken J. S. 1994, Chen S. M. 1994. JCM, Fishbein D. B., 1987, Maeda, K., 1987). Ehrlichia canis, the prototype species of the genus, is the etiologic agent of canine ehrlichiosis, actually only one of five Ehrlichia species that naturally infect dogs. Canine ehrlichiosis is a worldwide disease transmitted by the brown dog tick, Rhipicephalus sanguineus (Groves M. G., 1975. Lewis G. E. Jr., 1977). Ehrlichia canis causes a mild transient acute febrile illness and may progress to severe illness and a fatal syndrome (tropical canine pancytopenia) (Buhles W. C., 1974, Greene C. E. and J. W. Harvey. 1984, Walker, J. S. 1970). Each year it costs millions of dollars for treating companion and working dogs infected with E. canis worldwide. Moreover, E. canis also poses a public health threat. Ehrlichia canis, or an antigenically indistinguishable organism, was isolated from a human recently (Perez M. 1996).
Understanding the genetic and antigenic composition of E. canis is essential for studying the pathogenesis of canine ehrlichiosis and developing an effective vaccine. Ehrlichia canis is closely related to E. chaffeensis genetically and antigenically (Anderson B. E., 1991, 1992, Chen S. M., 1994. Am J. Trop Med Hyg). Therefore, canine ehrlichiosis may be an appropriate model for studying the pathogenesis of monocytotropic Ehrlichia spp. including E. chaffeensis.
The prior art is deficient in the lack of cloning and characterization of immunoreactive gene of Ehrlichia canis. Further, the prior art is deficient in the lack of recombinant protein of such immunoreactive gene of Ehrlichia canis. The present invention fulfills this long-standing need and desire in the art.